Pope Francis on Wednesday strongly decried the treatment of migrants crossing the Mediterranean Sea to enter Europe, saying it was a "grave sin" not to offer aid to migrant vessels.

"There are those who work systematically and with every means to reject migrants," the pontiff said during his weekly general audience in St. Peter's Square.

"And this, when done with conscience and responsibility, is a grave sin," he said.

The pope has spoken frequently about the treatment of migrants over his 11-year papacy. But his words on Wednesday, invoking Catholic terminology for one of the worst kinds of sin, were especially strong.

Migrants crossing the Mediterranean Sea in simple crafts or home-made dinghies from northern Africa and the Middle East have been the subject of intense debate across Europe over the past decade.

The International Organization for Migration estimates that more than 30,000 migrants crossing the Mediterranean have gone missing since 2014.

In Italy, a rescue ship operated by the Doctors Without Borders charity was issued a 60-day detention order on Monday. Authorities said the vessel, which had conducted several rescue operations on Aug. 23, failed to properly communicate its movements.

Doctors Without Borders refuted those claims. "We have been sanctioned for simply fulfilling our legal duty to save lives," it said in a statement.

Francis on Wednesday called for expanding access routes for migrants and a "global governance of migration based on justice, brotherhood and solidarity." The pope said the issue would not be resolved through the "militarization of borders".

In recent weeks, the pope had been offering a series of reflections about Catholic spiritual matters in his weekly audiences.

At the beginning of Wednesday's remarks, the pope said he was postponing that series this week, to consider "people who are crossing seas and deserts to find a place where they can live in peace and security".

Wednesday's audience was the last before Francis, aged 87, embarks next week for an ambitious four-country visit across Southeast Asia from Sept. 2-13. It is the longest trip yet by the pontiff, who now regularly uses a wheelchair due to knee and back pain.